To help ensure that population density estimates were representative of actual densities, we radio-tracked 28 Trioceros deremensis and determined that they spend approximately ¼ of their time too high in the canopy for us to see in our surveys. We adjusted density estimates for species with similar habitat use to T. deremensis accordingly

Research into chameleons’ sensitivity to landscape alteration in East Africa shows that species vary in their response. Some species increase in population density as a result of habitat fragmentation, partially counteracting the population decline from habitat loss.

The Eastern Arc Mountains (EAM) of Tanzania and Kenya are a chain of 13 forested mountain blocks sometimes referred to as the ‘Galapagos of Africa’ due to their remarkable biodiversity and endemism. This is particularly true of the EAM herpetofauna, which includes at least 99 endemic or near endemic amphibians and reptiles. While one third of amphibians are endangered, only a handful of the reptiles have been assessed (~13%) and half of these are critically endangered or endangered.

Most of these taxa are closed-forest specialists, which is a concern given that the EAM have lost ~80% of their historical forest, and the remaining forest is highly fragmented. This land use change is a central cause of decline for many of these species. However, our recent work in the East Usambara Mountains, supported by the National Geographic Society and the US Fulbright Fellowship program, highlights that species likely vary in their sensitivity to this threat.

Transect surveys

To assess species’ responses to habitat change, we surveyed chameleons in one of the EAM blocks, the East Usambara Mountains. As one may suspect, locating chameleons in a rainforest poses a challenge. Rather than attempting to locate every individual in an area (an impossible task!) we used distance-based transect surveys that allow for the estimation of detection probability. In this way we could get accurate estimates of population densities quantitatively accounting for the chameleons we could not see.

Although locally abundant in undisturbed habitat, Rhampholeon temporalis (above) respond negatively to habitat fragmentation. Conversely, Kinyongia vosseleri (below) do well in small, disturbed forest fragments, and may benefit from increasing habitat fragmentation. Both species are collected for the pet trade, but neither has been assessed for the IUCN Red List

For one of the larger species we also used radio telemetry to account for the time that they spent high in the forest canopy where we could not see them. We repeated these surveys across a landscape that has been heavily fragmented by logging, tea plantations, and small-scale agriculture. In total, we estimated densities of three species of chameleon in 12 forest fragments that varied in size, shape, and isolation.

Responding to deforestation

An important step in determining species’ sensitivity to landscape alteration is to quantify their responses to particular aspects of landscape alteration. In this case, chameleon densities respond to a change in forest fragment area. This relationship can compound, or alleviate, the effect of habitat loss on a species. If a species requires large blocks of forest to maintain healthy populations, their population density is likely to decrease in small habitat patches. Conversely, if a species prefers edge habitat to the forest interior, their density is likely to increase in small forest fragments. In both of these cases, the configuration of the remaining habitat is very important.

For the three species we examined, one shows a strong negative response to decreasing fragment area, one shows a weak negative response, and one shows a weak positive response – it actually has higher population density in small fragments.

Estimating declines of total population

While understanding how species respond to landscape configuration is interesting in its own right, it also provides a tool to estimate regional-level declines (or increases) that have resulted from recent land use change. To do this, we developed GIS maps of forest cover for the East Usambara Mountains – a large proportion of these three species’ ranges. This allowed us to quantify how much forest cover has been lost and also how the remaining forest cover is distributed – ie how big each remaining forest fragment is.

Using the relationships we had established between forest fragment size and chameleon densities, we estimated the entire population size in the East Usambara Mountains for each of the three species and compared that estimate to the expected population size if no deforestation had occurred.

For each species, we extrapolated population densities across the East Usambara Mountains of Tanzania. This map shows the estimated density of Rhampholeon temporalis for a portion of the landscape. Fragments surveyed for chameleon density are outlined in black

All species experienced an overall population decline driven by high levels of deforestation. However, Rhampholeon temporalis and Trioceros deremensis’ estimated declines (over 50% from historic levels) are magnified by their sensitivity to fragmentation effects beyond simple habitat loss. Conversely, Kinyongia vosseleri, which responds positively to small forest fragments, has likely declined less than one third from historical levels – less than would be expected from deforestation.

Although developing these population models for each individual species provides valuable information for conservation and management, they are also labour and resource intensive. If we are able to predict how species will respond to habitat alteration, conservation efforts can be concentrated on the most vulnerable species.

With support from the Mohammed bin Zayed Species Conservation Fund, our next step is to expand upon our study of chameleons in the East Usambaras to look more broadly across taxa and mountain blocks to examine the degree to which differences in species’ traits can explain variation in response to habitat change. Hopefully these efforts will yield a valuable conservation tool as well as ecological insights into how species’ traits influence their interactions with the landscape.